The present invention relates to an apparatus and a method that measure mean arterial blood pressure of a patient, and, more specifically, that provide noninvasive continuous recording and analyzing of the rate of impedance changes in two sections of the patient's body in order to continuously track mean arterial blood pressure. Still more specifically, the present invention relates to a method for continuously and noninvasively measuring both mean arterial blood pressure, left cardiac work index, and systemic vascular resistance index, utilizing an apparatus capable of miniaturization.
Mean arterial blood pressure (MAP) and cardiac index (CI) together define the forces and mechanisms involved in the circulation of blood through the cardiovascular system of a body. Measurement of MAP and CI when a patient is at rest (i.e., when a patient's body is in an inactive state) determines whether a patient has normal or abnormal blood pressure and blood flow. For example, MAP values indicate whether a patient has low blood pressure (hypotensive), normal blood pressure (normotensive), or high blood pressure (hypertensive), and CI values indicate whether a patient's blood is in a low, normal, or high flow state. Measurement of MAP and CI provides invaluable clinical information for "quantifying" the extent of blood circulation abnormalities, indicating the optimal course for therapy, managing patient progress, and establishing checkpoints for rehabilitation in a patient in whom fluid status control is essential. In addition, MAP and CI measurements define other important blood circulation information and mechanisms, such as oxygen transport characteristics of the cardiovascular system. For example, multiplied by CI, multiplied by a constant (i.e., LCWI=MAP.times.constant) LCWI directly relates to the oxygen consumption of the pumping muscles in the heart. The systemic vascular resistance index (SVRI) is approximately equal to MAP multiplied by a constant, divided by CI (i.e., SVRI=(MAP.times.constant)/CI SVRI is inversely proportional to the global oxygen demand of a body, and also represents a major component of the afterload on the heart.
For many diagnostic purposes, a resting measurement of MAP is important for determining the condition of a patient's cardiovascular system. A normal cardiovascular system is characterized by sufficient flow of blood to all parts of a patient's body, especially the brain and cardiac muscle, without producing prolonged strain on the physical capabilities of various organs through which blood flows.
In an abnormal cardiovascular system, blood pressure may be too high or too low, with each abnormality having attendant consequences for various body parts. Prolonged high blood pressure (hypertension) strains various organs in a patient's body and may end in heart failure, a cerebrovascular accident (stroke), or kidney damage. Knowledge that a patient is hypertensive informs a clinician to administer certain drugs and place the patient on a specified diet (e.g., one with a reduced sodium intake) to control the condition. Also, it can aid a clinician in discovering tumors or diseases that have afflicted a patient and that have caused the hypertension. Prolonged low blood pressure (hypotension) is typical when a patient has undergone hemorrhaging, through an accident or surgery. Hypotension can reduce the flow of blood to all parts of a patient's body, most seriously the brain and cardiac muscle, causing irreparable damage to those parts. Knowledge that a patient is hypotensive informs a clinician to use methods to raise the blood pressure of the patient.
Because blood is electrically the most conductive substance within any body segment, electrical bioimpedance measurements permit quantification of blood flow as a result of changes in electrical conductivity in a body segment. For example, the electrical impedance technique used for measuring cardiac output is based on changes in thoracic electrical impedance caused by cardiovascular activity. The impedance changes are measured by causing the flow of a fixed frequency constant magnitude current across a segment of a patient's body and sensing a voltage that is directly proportional to the instantaneous impedance. A number of devices have been developed to measure the impedance changes in body tissue resulting from blood flow, and correspondingly can accurately measure cardiac output (CO). However, none of these devices use bioimpedance techniques to measure blood pressure.
Thus, there is a need for a device that can accurately and continuously measure MAP through bioimpedance techniques so that one homogeneous technology can be used to measure and calculate MAP, LCWI, SVRI, and CI. The device can be miniaturized using microelectronic circuitry.
The need for such a bioimpedance device is evident from the fact that current methods for measuring arterial blood pressure are all based on the sphygmomanometric principle. In a typical sphygmomanometric measurement, an inflatable cuff is wrapped about a patient's upper arm and inflated so that it presses in on the arm. A determination of the systolic and diastolic pressure of a cardiovascular system is then made manually or automatically by monitoring the heartbeat of the patient as the pressure in the cuff decreases over time. In combination with an oscilloscope, the sphygmomanometric technique can measure MAP, but typically MAP is estimated from systolic and diastolic pressure by the following formula: ##EQU1## where P.sub.systolic is the systolic blood pressure and P.sub.diastolic is the diastolic blood pressure. Thus, current methods for measuring MAP provide mere estimates, which may not be accurate enough for a clinician to properly diagnose problems in a patient's body.
From a practical point of view, use of sphygmomanometry involves four additional drawbacks. First, it determines the average blood pressure over a plurality of heartbeats, and thus is in reality a series of tests. In addition, because systolic pressure is measured first, followed by a diastolic pressure measurement after a passage of time, the measurements are unlikely to correlate to the actual respective pressures at a given time. For example, the diastolic pressure may be different at the time the systolic pressure is measured, or the systolic pressure may be different by the time the diastolic pressure is measured because of the time delay between the two measurements. Second, the inflatable cuff hinders the flow of blood through the extremity to which it is attached (i.e., a patient's arm), and thus reduces the flow of blood to the portion of the extremity on the side of the cuff opposite from the heart. An adverse result of this reduction in blood flow is that ulnar nerve injury might occur if the measurement of blood pressure is repeated too frequently. Third, automated equipment for measurement of blood pressure requires pneumatic pumps and control valves that are bulky and have high power demand. Fourth, because of its size while inflated, the cuff limits the physical activity of a patient while a measurement is being taken. Also, if the cuff is connected to a device that automatically measures blood pressure, a patient's activity is further limited because the cuff is connected to bulky, possibly stationary pneumatic pumps and control valves.
Thus, there is a need for a device that can accurately and continuously measure MAP without hindering blood flow to any part of a patient's body or limiting the physical activity of a patient while MAP is being measured.